1. Field of the Invention
This invention relates to control of fugitive respirable dust in a method of transporting and crushing rock such as metal ore by spraying such crushed rock with a specific dust control palliative to control dust, where the method preferably involves a flotation circuit to concentrate metal, and where the palliative does not harmfully interact with reagents used in the flotation circuit.
2. Description of the Prior Art
Copper, lead, zinc, nickel, antimony, cadmium, molybdenum, vanadium and mercury are metals principally extracted from sulfide minerals; molybdenum mostly in the U.S. and Mexico, copper mostly in the U.S., Chile, Canada, Africa and some other countries. These minerals generally occur in relatively narrow veins necessitating the removal of large quantities of waste rock during mining operations, where the metallic content of the rock is low, about 0.25 to 3.0 wt. %. In the U.S., copper mining sites are mostly in western states, such as Utah and Arizona, where water necessary to metal recovery is in short supply and is in a process of conservation.
In all instances copper and the like ore, after mining transport, dumping onto conveyor belts, and multiple crushing to reduce particle size, is then transferred to a concentration means, such as an impeller type flotation machine. In the floatation machine, a variety of additives have been used in the past, such as pine oil, cresylic acid or amyl alcohol frothing agents; potassium ethyl xanate, sodium diethyl dithiophosphate or oleic acid “collecting agents” which additives film the particle so they adhere to air bubbles. Optionally, lime, soda ash, copper sulfate or sodium cyanide inorganic chemicals have been added in the flotation machine to prevent or assist filming the particles of the valuable minerals, which are carried by bubbles into a froth layer which is skimmed off, as described in detail in Rogers Industrial Chemistry (“RIC”), Ed., C. C. Furnas, Sixth Edition 1942, D. Van Nostrand, pages 914-930. An outline of treatment of low grade sulfide copper ore, at that time, is shown on page 923 of RIC, as is an impeller type flotation machine on page 919. Other type flotation machines are shown in Unit Operations of Chemical Engineering, W. L. McCabe and J. C. Smith, McGraw Hill, 1956, pages 382-384 as well as on page 919 of RIC.
With particular reference to copper ore, the objective of copper mining is to recover pure copper by extracting the mineral from coarse copper bearing ore. Copper bearing ore is removed from open pit mines by drilling, blasting, excavating, and crushing low grade ore. The ore is taken, usually by haul truck to primary crushers then via overland belts to secondary crushers. Along the way, raw ore is sometimes sent to stack out areas where it can be stored and then reclaimed in order to balance the flow of material between the excavating group and the ore concentration group. After the crushing process, coarse ore can go to a heap leach for chemical recovery of pure copper and is referred to as “oxide” ore. Or the coarse ore can be further crushed, and the fine copper ore sent to a froth flotation process for concentrated copper recovery and is referred to and herein defined as “sulfide” ore.
Sulfide ore contains between 0.25% and 5.0% copper metal with the balance being unwanted rock. Average copper metal content in copper ore is around 0.6%. In concentrating froth flotation circuits, crushed sulfide ore is mixed with thiol based chemicals such as but not exclusively xanthates which are referred to as “collectors”. Collectors coat the fine copper sulfide particles, known as chalcopyrite (CuFeS2) with a hydrocarbon coating that makes the chalcopyrite hydrophobic, that is, having little or no affinity for water—hydrophobicity. Raising the pH of the solution above ph 9.0 assists in the coating process. In the same process equipment, a “frother” is introduced which is generally an alcohol based non-ionic surfactant. Air is also either dissolved or induced into the flotation cell. The frother in contact with the air produces a swarm of bubbles which rises to the top of the froth cell. As the fine bubbles rise they preferentially come into contact with the hydrophobic chalcopyrite relative to the surrounding dirt and rock particles. The chalcopyrite concentrates in the foam and is skimmed off the top of the froth cell where it is sent to other processes to improve concentration and purity. Typical copper metal content of the froth cell foam concentrate is 20% to 40%. One type prior art cell is shown in FIG. 1.
The choice of surfactant for the frother and the application of the frother are closely guarded secrets within the copper mining industry, because the size of the foam bubbles and the surface tension of the bubbles can have a significant effect on the amount of copper metal recovered and the purity of the concentrate both of which have a large impact on the ultimate copper metal yield and the profitability of the copper mine.
Mine owners are reluctant to apply any chemicals to the raw copper ore, particularly surfactants, because of the potential to interfere with the flotation circuit. Very importantly, it is known that unwanted surfactants on the raw ore can alter the surface tension of the foam in the flotation circuit and/or create larger non-productive bubbles that collapse prior to skimming or hold less copper metal in the bubble surface. They can also resolubilize a portion of the collector back into the water phase resulting in poor copper recovery.
After concentrating, the concentrates (about 20-30 wt. % Cu or the like as Cu Sulfides, Fe Sulfides, other Sulfides and Silicates) have in the past been roasted, to adjust the proper ratio of Cu to S before fusing, and then passed through a process such as a reverberatory furnace smelting processor, to provide 30-45 wt. % Cu or the like “matte”, and a slag waste containing 0.2-0.5 wt. % Cu. Then a converter operation can be utilized to provide 98 wt. % Cu or the like, which is refined and cast into 99.5+ wt. % Cu or the like Anodes.
The process of mining, hauling, conveying, and crushing all ores, including copper bearing ore generates vast amounts of dust. The dust is unwanted by mine operators for the following reasons:                1. Dust particles that leave the transport process are no longer available for copper recovery therefore the cost of mining this fraction of the total has been lost.        2. Airborne dust particles contribute to air pollution and may result in the mine operator exceeding State and Federal air quality limits resulting in fines and a possible halt to production.        3. Airborne dust particles can be breathed in by mine workers, particularly in enclosed areas, which can lead to respiratory problems for workers as well as OSHA and/or MSHA fines to the mine operator.        4. Dust particles that escape conveyors and crushers in enclosed areas can accumulate in work areas causing an unsafe work area and contributing to higher maintenance and operating costs.        
Historically, mine operators have used local, regular water as a dust control palliative because it was inexpensive, readily available, and provided partial relief from the problems listed above. Although water is capable of wetting large pieces of copper ore, 0.32 cm to 15.25 cm (⅛″ to 6″), smaller particles, particularly below about 0.015 cm (100 mesh), are very difficult to wet with water and these are the very same particles that leave the process equipment and contribute to respirable dust. Because of the high surface tension of water and the low mass of dust particles, the dust particles are repelled. Adding excess water in order to wet these particles results in pockets of saturated ore that can stick to chutes and other conveying and processing equipment resulting in plugged equipment and unscheduled outages.
More recently, water reserves, particularly in the Western United States have become scarcer and consequently more expensive. Mine operators have discovered that adding excess water to control process dust is now expensive, causes operational and maintenance problems, is marginally effective, and may not even be possible in areas where water consumption is regulated.
There are many modern methods to produce copper from copper ore including, for example, cupric chloride leaching as taught by Clevenger et al. (U.S. Pat. No. 4,384,890); a chloride hydrometallurgical process taught by Satchell Jr. et al. (U.S. Pat. No. 4,594,132); a direct electrowinning process taught by Marsden et al. (U.S. Pat. No. 6,972,107); and medium temperature pressure leaching as taught by Marsden et al. (U.S. Pat. No. 7,341,700). However, little attention seems to have been paid to improving fins dust control while not aggravating process conditions downline.
In a completely different area, coal fines have been sprayed with a tall-oil-based emulsion to effect a chemical change in the coal, with the purpose to produce a synthetic fuel, as taught, for example, by Donovan and T is (U.S. Pat. No. 7,147,679).
There is a long felt need, over the last 70 years, for a way to conserve water in the above described mining processes. Even a 10% reduction would provide advantageous economics and potential compliance with potential future conservation efforts. Because of the negative impact to the sulfide ore flotation process discussed earlier, copper mine operators have been reluctant to use surfactants, and most anything except water, to assist in process dust control. As of 2008, no copper mine in the United States is known to commercially use surfactants for the purpose of process dust control.
Therefore, it is an object of this invention to provide a process to control dust particles during crushing and transporting rock, to reduce air pollution and protect workers.
It is a further object of this invention to provide a process during ore pre-processing that conserves water. It is a further object of this invention to spray dust particles during ore processing with a dust control palliative, which palliative will not provide harmful action on downstream flotation circuits or reduce metal production from the ore process.